Friday, July 06, 2012

One of the ways you can control how many mirror copies ASM keeps on disk is via diskgroup templates.

This presents an interesting question -- can you turn your normal redundancy disk group into a high redundancy by using a diskgroup template with high protection attribute thus telling ASM to use triple mirroring for related files?

...looks like the answer is no. It still sees that the remaining disk is missing required partners and does not allow the disk group to be mounted. How many extent copies the datafile had becomes a moot point.

The conclusion is that triple-mirroring inside a normal redundancy disk group can help you in certain cases (like physical corruption of both primary and secondary extents so ASM can use a third one to read the data) but it's not a substitute for a high redundancy disk group.

Update a little bit later: if anyone is thinking about doing it the other way around then remember that mirroring attribute works only for normal redundancy disk groups.